Method and apparatus for continuous plating



July 3, 1956 w. R. GRAY 2,753,298

METHOD AND APPARATUS FOR CONTINUOUS PLATING Filed Oct. 2, 1952 INVENTORW. R. GRAY,

BY KA MW ATTORNEY United States Patent METHOD AND APPARATUS FORCONTINUOUS PLATING William R. Gray, Sarver, Pa., assiguor to PittsburghPlate Glass Company, Allegheny County, Pa., a corporation ofPennsylvania Application October 2, 1952, Serial No. 312,807

7 Claims. (Cl. 20428) This invention pertains to electroplating, andparticularly to a method and apparatus for continuous copper plating,having special features which are adapted for continuously applying aprotective copper plating to the silver deposit on mirrors.

The continuous electroplating of metallic plates, sheets or similarshapes has been accomplished in various ways, and many efforts have beenmade to adapt such systems to the application of a copper layer upon thesilvered sides of mirrors. In the manufactureof mirrors, sheets of plateglass are silvered on one side by methods well known in the art, and theresulting silver layer is extremely soft and sensitive to abrasion aswell as chemical deterioration upon exposure to atmospheric elements.Even the application of protective varnishes, which usually havesubstantial viscosity, involves considerable chance of mechanicaldeterioration of the silver deposit, and thus the manufacture of highquality mirrors has been complicated and rendered unduly expensive byreason of the care which must be exercised with reference to the silverlayer, or by reason of the considerable number of rejects which areproduced.

It has therefore become common practicein the mirror manufacturingindustry to apply to the freshly deposited silver layer a protectivelayer of another metal, copper being the usual choice. Such a copperlayer can readily be formed by electroplating, and with comparativelylittle chance of damaging the silver reflecting layer.

One great objection to the copper plating of mirror surfaces has beenthe fact that the pieces being plated have had to be handled asindividual articles, one or more being positioned in the plating tankand electroplated and thereafter removed from the tank for furtherprocessing. This procedure has the usual undesirable features ofintermittent or batch operations, including the use of considerableskilled labor and the requirement for a sufficient number of separateplating tanks and other machinery to handle the desired production rate.

As a result of the above situation, mirror manufacturers have longsought for some process or apparatus which would enable them to achievethe desired copper plating in a continuous manner, but such efforts havebeen unsuccessful because of the difliculties in handling the fragilesilver layer carried .bythe glass sheets, as well as the difiiculty ofcontrolling the rate of copper deposition so that a uniform coat ofsufficient thickness is obtained.

It is therefore a principal object of the present invention to provide amethod and apparatus for the continuous electroplating of a protectivemetallic layer upon the silvered side of partially completed mirrorsheets.

A further object of the invention is to provide a method and apparatusof the type indicated in which the desired coating of protective metalis obtained at a speed sufficiently great to enable a single unit ofapparatus to pace existing commercial silvering production lines.

Still another object of the invention is to provide a method andapparatus for the continuous copper plating of the silver deposit onmirrors or the like, without injury 2,753,298 Patented July 3, 1956 tothe silver layer, whereby a maximum output of mirrors of uniformly highquality is assured.

Still another object of the invention is to provide a meth- 0d andapparatus of the kind indicated above which will permit efficientutilization of the chemicals utilized in the plating process.

With the above and other objects in mind, the invention will best beunderstood from the following detailed specification of the methodemployed, and of one preferred form of apparatus for carrying out themethod, reference being had to the appended drawings, forming a parthereof, and in which:

Fig. 1 is a side elevation, partly broken away and partly schematic, ofa preferred apparatus in accordance with the invention,

Fig. 2 is a fragmentary sectional view taken on the line 2-2 of Fig. 1,and

Fig. 3 is an enlarged isometric view showing one of the cathode rollersof the previous views and a preferred means for mounting the same in theapparatus.

Briefly, the method of the present invention depends for its successupon maintaining a relatively thin layer of the plating solution incontact with the silver film which .is to be plated, and passing theplating current from an anode which defines one surface of said thinfilm of electrolyte to the silvered surface, which then completes areturn path to the source of current. The return current is collected bymaking contact to the silver layer in such a Way that no sliding contactis involved, such as might result in damage to the silvered layer.Inasmuch as only a small quantity of plating solution or electrolyte isactually in use at any one instant, due to the thinness of theelectrolyte film, it is necessary that the same be continuouslyreplenished. Therefore, the method also contemplates the continuouscirculation of plating solution, so that the chemical composition of thepart represented by the thin film can be kept relatively constant.

The plating solutions employed may be those ordinarily used in knownintermittent plating processes. Thus, such a conventional solution maycontain 10% of copper sulfate and 1% sulfuric acid, by weight, thebalance being water. Such a solution has a pH of about 2. As is wellknown, the acid content may be reduced, and the pH thus raised, toreduce the corrosive effect of the solution on the Pounds Copper sulfate50 Rochelle salts 60 Triethanolamine 45 Water to make gals.

Such a solution will have a pH of about 7, and has the advantage thatthe corrosion problem is greatly simplified.

In accordance with the invention, the plating solution is maintained ina film about inch thick on the silver layer. This is accomplished bysupporting the silvered glass sheet in a horizontal position formovement beneath a transverse copper anode so arranged that a space ofabout /s inch is provided therebetween, and the plating solution isflowed upon the surface of the glass sheet in such quantity so that itfills the space between the sheet and the anode. An excess of solutionis supplied, and the run-ofi can readily be collected and recirculatedthrough a reservoir of such capacity that the composition of.thesolution varies only slightly with time. Fresh solution "or make-upchemicals may of course be added from-time to time,-as required.

It was found that silvered sheets of commercial size, for example, aswide as 96 inches in the directionof anode length, could be treatedat-satisfactoryspeeds-by'moving the silvered sheets continuously beneaththe anode in such a way as-to maintain the thickness of the film atapproxi- 'mately inch, as indicated above.

"However, it was also found that while the rate ofdeposition couldbeincreased by increasing the dimension of the anode in the direction ofplate travel, this dimension must be kept to a value not over 20 inches.The use of wide anodes resulted in inability to maintain the solution asa continuous film, so that certain portions of thesilver-layer receivedinsufficient copper deposit.

The return path for the plating current is provided by a multiplicity ofhighly polished copper cathode rollers Time,

I Copper seconds Deposit, Gms./sq.' it.

Amperes Inasmuch as a satisfactory copper film requires an averagedeposit of the order of 0.1 gram of copper per square foot, a practicalapparatus capable of maintaining a commercially satisfactory linearspeed requires a somewhat wider anode, a larger current, or the use ofmore than one anode. A preferred embodiment of commercial apparatus forcarrying out the method is shown in the appended drawings, to whichreference is now made.

The drawings illustrate, partially in schematic fashion, a preferredembodiment of an apparatus by which the novel method may conveniently bepracticed. InFig.

1, there is shown a plurality of spaced-apart rollers 10,

which may be coated with rubber or like resilient material, and arrangedwith their axes lying in a common horizontal plane so as to form asupport for a sheet or sheets of glass 12, these sheets having on theirupper surfaces the silver reflecting coating needed for mirroroperation. In Fig. 1, the sheet or sheets 12 travel from left to rightas indicated by the directional arrow, and

are propelled by any conventional means.

In the apparatus shown, there are two anodes and two sets of cathoderollers, the rollers of each set being carried by transverse beams 14and 16, and the rollers themselves being shown at 18. Each roller isformed as a highly polished disc of copper, and the rollers comprisingeach set are maintained substantially in coaxial alignment by means tobe described. Inan apparatus suitable for handling sheets 12 of theorder-of 48 inches wide, for instance, there may be eighteen oftheserollers 18, spaced about 2 /2 inches apart.

The anodes in Fig. 1 are shown as flat plates having their leading andtrailing edges upturned, this construction aiding in the maintenance ofan uninterrupted film of electrolyte or plating solution on the glasssheets 12. As shown, the anodes 20 are located and suitably 'supportedso that their under surfaces "are about /8 inch "above the surfacesbeing plated, and they extend (in the tance at least equal tothe widthof saidsheets.

are two of these copper anodes in the apparatus shown,

indicated in the drawings.

and two sets of copper cathode rollers 18, one set of rollers beingdisposed in advance of the first anode and the second set being disposedbehind the last anode.

A tank 22, which may p'referably be disposed above the table rollers 10for gravity feed, contains a quantity of the copper plating solution orelectrolyte, which is fed through a conduit 24 to a distributing pipe 26extending across the apparatus, and thence via pipes 28 and 30 to theupper surfaces of the sheetsbeing plated. As shown, the distributionpipes may include valves for controlling the rate of feedingof the.solution, and the lower ends of said pipes preferably terminate atpoints just ahead of the respective anodes 20. There are as many ofthese pipes 28 and 30 as required, widthwise of the machine, to ensure asuflicient supply of the plating solution for film formation beneathanodes 20.

At the right hand end of the machineanupper roll 32 is provided, andpreferably coveredwith a rubber layer-or the like, to act as a squeegeeand thereby to prevent excess solution from being carried beyond thedesired point. A tank 34 is disposed beneath and partially encompassingtable rolls 10, and solution leaving the plated sheets 12 runs into thistank and thence into a collector pipe 32 from which it is recirculatedby pump 38 and conduit 40 to the tank 22.

The arrangement of the cathode rollers '18 across the machine is bestillustrated in Fig. 2 which also illustrates one way in which the tablerolls 10 can be disposed'between side'frarnes'which also carry the tank34.

In order to prevent physical damage to the sensitive silversurface'entering the plating apparatus, the cathode rolls 18 whichcontact this surface are arranged for selfalignment in a'mannerbestshown in Fig. 3, which shows one of these rollers and its supportingmeans in enlarged perspective view. In that figure, a representativeroller 18 is illustrated as carried by cross beam 1'4 by a sort ofuniversal connection comprising a yoke 42 in which roller 18 is.journalled as by bearing pins 44, whose axial positions may be adjustednicely (for free rotation of the roller) and secured by set screws orthe like. Each yoke 42 is similarly pivoted for horizontal swingingmovements in a second yoke 46, as by bearing pins 48 similar to those ofyoke 42, and this second yoke 46 is in turn carried by a yoke block 50in pin bearings 52 for free vertical swinging movement.

Each block 50 is mounted for vertical adjustment in a block 54, as by adovetail arrangement 56, and the vertical position of each block50 canbe adjusted by means of a screw operating in a threaded "hole in a lugcarried by a pillar 58 secured to block 50, said screw 60 having its endin engagement with the upper surface of block 54. In thisway, thevertical position of each block 54 can beset as desired for equalitywith the other rolls 18 in a set, and the roll 18 will be free forslight movements vertically and horizontally so that a minimum of dragwill be placed on the silver surface beneath each roller.

To ensure good electrical contact between the roller 18 and its supportbeam or bar 14, a conductive jumper 62 may be connected between block 54and yoke 42, as Theplating current is then connected to the anodes 20and the cathode rollers 18 in the known way.

In one embodiment of v a commercial machine, the anodes 20 .have a widthof 8 inches and are approximately 96 inches long and extend entirelyacross the mirror plates, and the linear speed of travel of the platesis about 36 inches per minute. This speed of travel could be increased,of course, by increasing either the anode area or the plating current,or both. Other modifications in the disclosed apparatus, and minorvariations inthe disclosed method, may be made-by those skilled in theart without departing from the spirit of the invention as defined in theappended claims.

What is claimed is:

1. The method of electroplating a metallic protective layer upon thesilvered surface of a mirror, comprising passing a silvered sheet ofglass continuously in a substantially horizontal plane beneath an anodeplate extending crosswise of said sheet and being of substantial widthas measured in the direction of sheet travel, the silvered surface ofsaid sheet facing said anode plate and being closely adjacent the same,continuously flowing plating solution into the space between said sheetand said anode plate only in sufficient quantity to maintain aconstantly changing unbroken single thin layer of solution filling thespace between the anode and the sheet, and passing a plating currentfrom said anode plate through said layer of solution and into the silversurface.

2. The method in accordance with claim 1, including the steps ofcollecting plating solution flowing off of said sheet and continuouslyrecirculating said solution into position between said anode and saidsheet,

3. Apparatus for the continuous electroplating of sil vered sheets toprovide a protective metallic coating over the silver layer, comprisingmeans for supporting and guiding a silvered sheet for movement in ahorizontal plane, with its silvered surface uppermost, a flat anodeelectrode extending both widthwise and lengthwise of said sheet andsupported in closely spaced parallel relation above the silvered surfaceof said sheet, a plurality of freely rotatable cathode contact rollersdistributed crosswise of said sheet adjacent said anode and swingablymounted for horizontal and vertical motion to rest lightly in contactwith the silvered surface of said sheet and to distribute platingcurrent substantially uniformly widthwise thereof, and liquiddistributing manifold means having outlets adjacent said anodedistributed crosswise of the sheet and above the same to direct acontinuous stream of plating solution onto the silvered surface and intothe space between said surface and said anode, in suflicient 6 quantityto maintain a thin unbroken layer of solution filling said space.

4. Apparatus in accordance with claim 3, including means for wipingexcess solution off of said sheet after passage past said anode, andmeans for collecting runoff solution and recirculating the same onto thesaid surface.

5. Apparatus in accordance with claim 3, in which the space between saidanode and the silvered surface of said sheet is of the order of 4; inch.

6. Apparatus in accordance with claim 3, including universal mountingmeans for said cathode contact rollers, for free horizontal and verticalmovements thereof upon said silvered surface.

7. Apparatus in accordance with claim 3, in which there are a pair ofsaid anode electrodes, spaced apart in the direction of sheet travel,and in which a set of said rollers is mounted adjacent each of saidanodes.

References Cited in the file of this patent UNITED STATES PATENTS Re.23,456 Rieger Jan. 22, 1952 936,472 Pfanhauser Oct. 12, 1909 1,115,671Hermann Nov. 3, 1914 2,244,423 Hall June 3, 1941 2,271,736 Hall Feb. 3,1942 2,345,356 Owen Mar. 28, 1944 2,372,599 Nachtman Mar. 27, 19452,372,665 Egli et al. Apr. 3, 1945 2,476,286 Cox July 19, 1949 2,490,055Hoff Dec. 6, 1949 2,512,328 Hays June 20, 1950 2,513,515 Powers July 4,1950 2,540,602 Thomas et al. Feb. 6, 1951 FOREIGN PATENTS 20,237 GreatBritain of 1899 345,676 Great Britain Mar. 27, 1931 331,930 France Aug.11, 1903

1. THE METHOD OF ELECTROPLATING A METALLIC PROTECTIVE LAYER UPON THESILVERED SURFACE OF A MIRROR, COMPRISING PASSING A SILVERED SHEET OFGLASS CONTINUOUSLY IN A SUBTANTIALLY HORIZONTAL PLANE BENEATH AN ANODEPLATE EXTENDING CROSSWISE OF SAID SHEET AND BEING OF SUBSTANTIAL WIDTHAS MEASURED IN THE DIRECTION OF SHEET TRAVEL, THE SILVERED SURFACE OFSAID SHEET FACING SAID ANODE PLATE AND BEING CLOSELY ADJACENT THE SAME,CONTINUOUSLY FLOWING PLATING SOLUTION INTO THE SPACE BETWEEN SAID SHEETAND SAID ANODE PLATE ONLY IN SUFFICIENT QUANTITY TO MAINTAIN ACONSTANTLY CHANGING UNBROKEN SINGLE THIN LAYER OF SOLUTION FILLING THESPACE BETWEEN THE ANODE AND THE SHEET, AND PASSING A PLATING CURRENTFROM SAID ANODE PLATE THROUGH SAID LAYER OF SOLUTION AND INTO THE SILVERSURFACE.